GESTIÓN RESPECTO DE LOS TRABAJADORES POR CUENTA PROPIA PROPIA

Basic levels of interaction were assessed in the system, by performing direct analyses of the dataset. There were clear relationships between some of the major sampling parameters, such as TSS and BOD, and related factors, such as amm N and TON. However, upon efforts made to analyse the data more deeply, it became clear that the analysis also showed that a number of interactions were not being identified by direct analysis.

168

As a result, an alternative method for analysis of interactions in the dataset was devised. Table 4.1 shows a map constructed from the ANOVAs of all measurable parameters of the BESST. The data was split into two sampling populations for each key variable (i.e. amm N, BOD and TSS). One group related to periods of ‘good’ treatment, one group related to periods of ‘bad’ treatment. Good or bad treatment was assigned according to whether consents had been met or not.

The section of Table 4.1 showing the analyses for the treatment of amm N contains the highest number of statistically significant differences at 68. TSS has 59 and BOD has 39.

Based on statistical analyses, the constructed ‘conditions map’ shows initially that each zone of the system is important, and that each parameter sampled interacts within the system to some extent. The map has been highlighted to outline the most significant parameters affecting good and bad treatment. From this, trends are identifiable. The analysis for the treatment of amm N contains the highest number of statistically significant parameters, with TSS the second most and BOD the third.

169

Amm N DO pH Temp Eh Cond Amm-N TON NO2- Tot-N COD Sol COD BOD Sol BOD Tot-P Orth-P VFA TSS VSS SVI AOB NOB Dn

Inlet + + - mv mv mv Anoxic + - - + - - - + Aerobic + - + - - + - - Clarifier B + - + - - + - - - - - - - - + - Clarifer M - + - + - - - - - Clarifier T + - + - - + + - - - - - Outlet + + + - - + - - - - - - mv mv mv

BOD DO pH Temp Eh Cond Amm-N TON NO2- Tot-N COD Sol COD BOD Sol BOD Tot-P Orth-P VFA TSS VSS SVI AOB NOB Dn

Inlet - - mv mv mv Anoxic + - - - - Aerobic + + - - Clarifier B - - + - - Clarifer M + - - + - + Clarifier T + + - - + + + - Outlet + - - + - - - - - mv

Solids DO pH Temp Eh Cond Amm-N TON NO2- Tot-N COD Sol COD BOD Sol BOD Tot-P Orth-P VFA TSS VSS SVI AOB NOB Dn

Inlet - - - + - - - - mv mv mv Anoxic - - + - - - + Aerobic + - - + - - - - + + Clarifier B - + - - mv - - - - + Clarifier M + - + - - + Clarifier T + - + - - mv - - - Outlet + + - - + - - - - - -

Table 4.1 Conditions Map, for the treatment of amm N, BOD and TSS. ‘+’ = when this variable is high, effluent quality is good. ‘-‘ = when this variable is high, effluent quality is poor. ‘mv’ = missing value

170

Some variables may interact with each other to a greater extent than they do with the rest of the system. For example, DO affected TSS (i.e. MLSS) in the aerobic zone and vice versa. Generally, the relative trend was the higher the DO, the lower the MLSS. This is an important aspect of sewage control. Within the map, there are some occurrences where DO showed a positive (+) value and TSS a negative (–) value (i.e. Clarifier B, Clarifier T and outlet on the amm N map), which is likely to relate to the immediate effect they had on one another. At other instances, when DO and TSS were highlighted as being significant separately from one another (i.e. Aerobic DO positivie and Clarifier M TSS negative on the amm N map) it was easier to assess their significance with relation to the requirements of the system and a steady-state equilibrium.

These aspects of treatment of amm N in the system can be justified – a higher DO in the aerobic zone is desirable because nitrifying populations found in the aerobic zone require an oxygenated environment to oxidise amm N. A lower level of TSS in the middle of the clarifier means less TSS will eventually rise to the top of the clarifier, and be released in the effluent. The carryover of solids which may be associated with high TSS in the clarifier would also contain biomass and amm N. Similarly, during periods for which negative values are assigned to consentable parameters (e.g. amm N, TSS, BOD), clearly such values should not necessarily be over analysed – good treatment of BOD does require low levels of BOD in the effluent, and that means being assigned a negative value.

This method, while being illustrative, does not detail the boundaries of the higher limits of these parameters. For example, too high a level of DO in the aerobic zone can precede a loss of MLSS, and eventual washout of the system. It is also

171

important to remember that the relationships identified by this table are relative, and based on variations over ranges of time.

Using the map it is also possible to highlight spatial trends within the data. On the amm N map, inlet pH was assigned a positive value, anoxic pH no value and the aerobic and clarifier pH a positive value. The blank anoxic value can be interpreted as a buffer zone, where a high pH is introduced into a low pH environment.

Eh is an important factor for all three major parameters. All aspects of the map favour higher Eh in all zones, which contradicts the basic requirements for

conventional anaerobic denitrification. The potential for an alternative method for denitrification in this system is indicated by these results, such as aerobic

denitrification. Conductivity appears to have the same level of impact as Eh, however, lower levels are favoured within the system. Low conductivity is generally a strong indicator of a healthy system as it reflects the presence of

dissolved inorganics (i.e. phosphates, nitrates, other anions and cations) in the bulk liquid. On the solids map, temperature and conductivity are both largely assigned negative values. Higher temperature affects conductivity, which increases in response, so this association may represent the inherent relationship between temperature and conductivity.

Levels of TP and PO4 have little apparent influence on the treatment of amm N. For

the treatment of BOD, a negative value is assigned to the inlet, the bottom and middle of the clarifier, and outlet. This may be indicative of an issue with dissolved inorganics. These parts of the clarifier are high sludge areas, and lower levels of P and PO4 in samples may indicate that less P and PO4 is present in the general liquid

172

system. Similarly, levels of VFA show a similar pattern – if the organic carbon source is at too high a level, there is a danger of overgrowth of heterotrophic bacteria. A consistent negative value throughout the system suggests that VFA is being utilised at the correct rate for maintenance of a steady state. As discussed before, this suggests that pH is an important factor in the treatability of amm N. Conversely, pH seems to be not as significant a parameter for the treatment of solids and BOD. This may be accounted for by the nature of the parameter undergoing treatment. Amm N oxidation and removal is largely a biological process, whereas the treatment of solids is also a mechanical or physical process. The treatment of BOD is also biologically mediated; but not as reliant on

microbiological processes as that of amm N and nitrification.

Amm N is assigned a negative value throughout all three maps. Generally, low amm N is a strong indication that a biological nutrient removal system is working well. This is demonstrated in the maps, as low amm N is important for the removal of TSS and BOD. TON and nitrite are closely related to amm N and are assigned positive values throughout the maps, suggesting a higher level of both nitrite and nitrate being important for the successful treatment of all three major parameters. A high level of nitrite does suggest more productive rates of ammonification within the system, although nitrite would be expected to be converted to nitrate quickly in the system and therefore, for this to be reflected in the conditions map. The

conditions map show few bacterial interactions for the treatment of amm N, but these results suggest that a considerable amount of ammonification may have taken place in the anoxic areas of the system.

The positive impact of relatively high DO in the Clarifier B and Clarifier T, between days 200 and 300, and a relatively higher DO in the Clarifier T,

173

corresponded with higher effluent amm N. At this point in the overall process, it can be assumed that clarification is the only process taking place, and biological activity is generally restricted to the anoxic and aerobic zones only. As a higher DO generally correlates with a lower TSS, it is possible that the positive relationship identified between high DO in the clarifier and low effluent amm N is coincidental in this sense.

Inlet pH was also identified as a factor affecting treatment of amm N. However, Table 4.1 shows that the pH is stabilised within the system to a much lower value than that entering the system in the influent. This is supported by a large drop in influent pH after day 600, which is not reflected in the aerobic zone. Despite this, the unusual drop in pH may still have had an impact on treatment, and a rise in effluent amm N at this time (day 625). For example, there may have been an unusually high proportion of fatty acids in the influent at that time, providing an added source of nutrients for heterotrophic bacteria, adversely affecting levels of nitrifying bacteria. However, a rise in levels of effluent amm N is likely to have been due to an ongoing problem, particularly as this period of poor treatment lasted until the end of the monitoring period. This situation identifies a limitation in the use of the conditions map. Since the inlet pH is always higher than the pH in the rest of the system, and for the majority of the sampling scheme levels of effluent amm N were within acceptable limits, a bias towards an overall positive effect is evident. This was highlighted by ANOVA testing, but is not necessarily

representative of a significant relationship.

The conditions map highlighted a relationship between good effluent BOD with high levels of TON in the clarifier. A higher TON suggests a level of activity of nitrifying bacteria that which would increase oxygen demand. However, the oxygen

174

demand from nitrifying bacteria is not detected by the BOD test, and so resulting in low BOD. If there is a large population of nitrifying bacteria, this may lead to an inhibition of growth of heterotrophic bacteria which would also reduce the BOD overall. However, it is not clear why this should take place in the clarifier, which is a very physically dynamic part of the system. The colonisation of the sludge blanket by nitrifying bacteria is not impossible, although it would be expected that the presence of filamentous bacteria there would prevent this from happening. An explanation for this is that because the TON in the clarifier was generally stable, and reflected what was occurring in the rest of the system, other aspects of the system which led to a high TON value in the clarifier also contributed to good effluent quality. This would be an extremely complex set of relationships to examine, however, there appears to be no obvious reason why high TON in the clarifier should contribute to good effluent levels of BOD.

A high DO in the aerobic zone was also preferential for good treatment of BOD. It is difficult to ascertain the direct effect of aerobic zone DO on effluent BOD until it is linked with MLSS. This was likely to have been highlighted by statistical

analysis because in the aerobic zone, high DO is often the result of lower MLSS. In this situation, low solids in the aerobic zone are translated to the effluent, which ultimately results in a lower effluent BOD.

Temperature was believed to be an important factor for the effects on the biomass of this system, as it was installed above ground. The impact of low temperature was observed largely during the treatment of solids in the system, and had only a slight impact on the treatment of amm N and apparently none on BOD.

175

TSS and VSS appear to have surprisingly little influence within the system on the effluent quality of solids. However, good amm N treatment requires relatively low TSS throughout the clarifier; which may be related to the carryover of solids, and subsequently biomass in the final effluent. The only assigned significance of SVI is assigned to the aerobic zone of the solids map, and is given a positive value. This was to be expected. Good settleability of sludge leads to reduced levels of solids carryover from the clarifier. Less carryover is also generally associated with a healthier biomass, an adequately aerated system and therefore also lower effluent TSS.

176

5. Discussion